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Fossil stromatolite, Barberton Mountains South Africa (2.5 billion years old)
Fossilization is a rare occurrence, because most components of living things tend to decompose relatively quickly. In order for an organism to be fossilized, the remains need to be covered by sediment, frozen, desiccated, or come to rest in an oxygen free environment. There is a large number of fossils of organisms that had hard body parts, covered large portions of the planet, or existed for a long part of history. Earth’s oldest fossils are stromatolites consisting of rock built from layer upon layer of sediment and other precipitants.
Fossils are created in several ways. The first, permineralization, occurs after burial, as the spaces filled with liquid or gas during life become filled with mineral-rich groundwater and the minerals precipitate from the groundwater, thus occupying the empty spaces. This process can occur on as small a scale as cell walls. Small scale permineralization can produce very detailed fossils. This is a form of diagenesis.
When the original remains of the organism have been completely dissolved or destroyed and all that is left is a shaped hole in the rock, it is called an external mold. If this hole is later filled with other minerals, it is a cast. An internal mold is formed when sediments or minerals fill the internal cavity of an organism. Authigenic mineralization is a special form of cast and mold formation. If the chemistry is right, the organism can act as a nucleus for the precipitation of minerals such as siderite, resulting in a nodule forming around it. If this happens rapidly very fine three-dimensional morphological detail can be preserved.
Replacement occurs when the shell, bone or other tissue is replaced with another mineral. In some cases mineral replacement of the original shell occurs so gradually and at such fine scales that microstructural features are preserved despite the total loss of original material. Recrystallization occurs when the original skeletal compounds are still present but in a different crystal form(aragonite to calcite).
Compression fossils are the result of chemical reduction of the complex organic molecules composing the organism’s tissues. The fossil consists of original material in an altered state. What remains is a carbonaceous film called phytoleim, in which case the fossil is known as a compression. The phytoleim is usually lost and all that remains is an impression of the organism in the rock. In many cases compressions and impressions occur together. When the rock is broken open the phytoleim will often be attached to one part (compression), whereas the counterpart will just be an impression.
Bioimmuration is a type of preservation in which a skeletal organism overgrows another organism, preserving the other, or an impression of it, within the skeleton. Usually it is a sessile skeletal organism, such as an oyster covering other sessile encrusters. Occasionally, the bioimmured organism is soft-bodied and is then preserved in negative relief as a kind of external mold. There are also cases where an organism settles on top of a living skeletal organism which grows upwards, preserving the settler in its skeleton.
We have written many articles about fossils for Universe Today. Here’s an article about what fossils are, and here’s an article about the Cambrian explosion.
If you’d like more info on fossils, check out the U.S. Geological Survey Homepage. And here’s a link to NASA’s Earth Observatory.
We’ve also recorded an episode of Astronomy Cast all about planet Earth. Listen here, Episode 51: Earth.
Sources:
http://en.wikipedia.org/wiki/Fossil
http://www.acad.carleton.edu/curricular/BIOL/classes/bio302/Pages/FossilizationHome.html
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